The present invention relates to a method of manufacturing an electroluminescent diode in which a first layer containing at least one element of column III-B and at least one element of column V-B of the period table of elements and having a higher content of isoelectronically incorporated nitrogen than the substrate is deposited epitaxially on a substrate, a second layer having a lower content of isoelectronically incorporated nitrogen than the first layer is deposited on the first layer and a p-junction is formed in the first layer.
The present invention also relates to optoelectronic devices manufactured with the material obtained by said method.
It is known that the above-defined so-called III-V semiconductor materials having an indirect band structure can produce a luminescence on the condition that in given sites which are occupied by atoms of the constituent of column V, especially phosphorus and arsenic, the said constituent is replaced by a nitrogen atom. The nitrogen substitution atoms surrounded by normal sites form what is called isoelectronic trapping centers; these may pass to the excitation condition and form intermediate levels within the forbidden band. The luminescence, for example the green luminescence of GaP, is caused by the de-excitation of the electrons trapped at said levels.
It is also known that the same materials are used for the manufacture of photoreceivers.
The number of efficient radiation recombinations, which is estimated by measuring the level of the cathode luminescence, is proportional to the concentration of said trapping centers. It has been found, however, that when the nitrogen content is low, such proportionality is true and that when the nitrogen content increases, there occurs first, a saturation in which the luminous efficiency increases more slowly than the nitrogen concentration, after which, after a maximum, the direction of the curve is inverted.
It is known that the most efficient diode, especially the green luminescent diode of gallium phosphide, is obtained by liquid epitaxy of one or several layers, because the epitaxy from the gaseous phase involves a large number of crystal defects.
It has also been proposed in the U.S. Pat. No. 3,646,406, patented on Feb. 29, 1972, to perform the epitaxy in the presence of an atmosphere containing ammonia. The ammonia molecules which contact the gallium solution at high temperature, are broken down, the surface of the solution serving as a catalyst, and active nitrogen atoms dissolve in the gallium and deposit on the crystal lattice in the phosphor sites during the growth of the epitaxial layer.
The n and p dopings which are necessary to form the junction are manufactured during the epitaxy by means of lead sulphide and zinc vapours, the junction being epitaxial.
The drawback of the formation in this manner of a p-n junction during the epitaxial deposition of the layers is that the incorporation of nitrogen depends on the impurity, so that the partial nitrogen pressure has to be adapted to in accordance with the change of the impurity.